1. Field of the Invention
The present invention pertains generally to devices, methods and systems used in radiation detection, and more particularly to position sensitive lithium-drifted detectors, particularly those utilized near room temperature.
2. Description of Related Art
Lithium-drifted Germanium (Ge(Li)) detectors were developed and successfully used for gamma-ray spectroscopy in the early 1960s. The excellent energy resolution and good efficiency achieved by these detectors provided experimenters with unprecedented capabilities in gamma-ray measurements, and this has revolutionized the field of gamma-ray spectroscopy. In the 1970s, high-purity Ge became available and rapidly replaced Ge(Li). With high-purity Ge, the detectors can be made stable at room temperature, and thus the fabrication of detectors can be carried out without the severe time constraints that existed when working with Ge(Li) detectors. This opened up new possibilities in terms of detector processing, and allowed the production of more diverse types of detectors and multi-detector systems.
In the ensuing decades, there have been continuing advances in semiconductor gamma-ray detector technology. Ge detectors continue to increase in size and complexity. More recently, detectors based on other materials, such as Si and CdZnTe, have also been actively developed for gamma-ray detection. Advances in the technology of semiconductor gamma-ray detectors are the results of contributions from many groups.
U.S. Pat. No. 6,486,476, incorporated herein by reference and attached hereto, discloses a semiconductor radiation detector and a method for fabricating such detector. U.S. Pat. No. 5,773,829, also incorporated herein by reference and attached hereto, discloses a scintillator-based radiation detector, which is somewhat representative of generic radiation detection.
FIG. 1 illustrates a lithium drifted silicon detector fabricated conventionally. The detector consists of a p-type silicon substrate having an n-type lithium contact on the top surface, and a p-type gold surface barrier contact on the bottom surface. The figure illustrates the “deep groove” configuration, but such a detector may be fabricated in the top hat configuration and with straight sides as well.
The method for fabricating a segmented lithium-drifted silicon detector using conventional fabrication processes comprises the steps of: (a) providing a p-type silicon substrate having a top and bottom surface; (b) forming an n-type contact by depositing a lithium layer on the top surface and diffusing it into the substrate by heating the substrate; (c) forming a p-type contact by chemically etching the bottom surface and then evaporating gold metal on that surface to form a surface barrier, (d) compensating the p-type substrate by simultaneously applying heat and an electric field between the n-type contact on the top surface and the p-type contact on the bottom surface for a sufficient amount of time to cause the lithium to drift into the substrate and compensate the p-type impurities to a residual concentration of less than or equal to 1010 cm−3 within the substrate, and (e) segmenting the lithium contact (n-type) either by chemical etching or mechanical removal (i.e. sawing) to remove material in order to isolate the conductive segments from one another.
In the conventional process, since the diffused lithium contact has a thickness of between twenty and several hundred microns, a significant amount of material must be removed. Although this is a viable process, it suffers from device pitch limitations (number of segments per mm). In the case of the gold surface barrier (p-type), the adhesion of the gold to the silicon surface is also very poor, making electrical connection to the substrate difficult and rendering the contact prone to failure.
Therefore, it will be appreciated that new fabrication methods are needed to simplify the fabrication process of radiation detectors and that new device structures are needed which lend themselves to new fabrication methods. The present invention fulfills these needs as well as others and overcomes limitations of prior radiation detection devices.